For purposes of diagnosis and treatment planning, imaging techniques are commonly used in medical procedures to view the internal anatomy of a patient's body. Although the technology for rendering real-time 3-D ultrasound images of most internal organs have been around for several years, real-time 3-D ultrasound in cardiology requires much higher frame rates for real-time acquisition and display to keep up with the beating heart or other cardiac motions. Until recently, 3-D rendered images of the heart have been generating on a non-real-time basis by sequentially acquiring two-dimensional and then using a workstation to input these images for volume rendering.
Recent advancements in transducer and processing technology have enabled commercially available real-time 3-D ultrasound imaging of the heart and surrounding vasculature. For example, the SONOS 7500 imaging system, marketed by Philips Medical System located in Bothell, Wash., is an example of one such commercially available system that uses an external device to generate the image. This system provides real-time 3-D images of cardiac structures with resolution that is adequate for assisting in catheter navigation and placement during electrophysiology procedures. See, e.g., Lang et al., “A Fantastic Journey: 3D Cardiac Ultrasound Goes Live,” Radiology Management, November/December 2002; and “Phillips Prepares to Launch System Upgrade Capable of True Real-Time 3D Echo,” Diagnostic Imaging Scan, The Global Biweekly of Medical Imaging, Vol. 16, No. 18, Sep. 11, 2002, the disclosures of which are hereby expressly incorporated herein by reference.
During electrophysiological therapy, ablation is used to treat cardiac rhythm disturbances. During these procedures, a physician steers a catheter through a main vein or artery into the interior region of the heart that is to be treated. The physician places an ablating element carried on the catheter near the targeted cardiac tissue that is to be ablated, and directs energy from the ablating element to ablate the tissue and form a lesion. Such a procedure may be used to treat arrhythmia, a condition in the heart in which abnormal electrical signals are generated in the heart tissue.
To some degree, a real-time 3-D imaging system, such as the SONOS 7500, obviates the need for a 3-D catheter navigation system. A 3-D navigation system, however, would still be very useful for correlation of catheter position and internal anatomical structures with previously recorded signals and ablation locations.
In one navigation system, commercially available as the Realtime Position Management™ (RPM) tracking system developed by Boston Scientific Corporation, located in San Jose, Calif. a graphical representation of a catheter is displayed in a 3-D computer-generated representation of a body tissue, e.g., heart chamber. The 3-D representation of the body tissue is produced by mapping the geometry of the inner surface of the body tissue in a 3-D coordinate system by placing plurality of ultrasound positioning transducers on a catheter, and moving the catheter to multiple points on the body tissue while tracking the positions of the catheter within the global coordinate system using the positioning transducers. A graphical anatomical shell is then deformed to conform to the transducer positions as they are acquired. The positions of other catheters to be guided within the body, e.g., a mapping/ablation catheter, is determined by placing ultrasound transducers on the these catheters and tracking the positions of the catheters within the 3-D coordinate system.
In the case of cardiac treatment, electrical activity sensed by the ablation/mapping catheter can be correlated with the sensed positions of the catheter in order to generate and register an electrophysiology map within the 3-D coordinate system. Tissue associated with abnormal activity, such as cardiac arrhythmia, can then be treated by guiding the ablation electrode of the mapping/ablation catheter into contact with the tissue, as shown on the electrophysiology map, and energizing the electrode to create a lesion on the tissue.
Recent work at Duke University has demonstrated the ability to localize catheters within a 3-D ultrasound image, such as that generated by the SONOS 7500 imaging system. See, e.g., Merdes et al., “Locating a Catheter Transducer in a Three-Dimensional Ultrasound Imaging Field,” IEEE Transactions on Biomedical Engineering, Vol. 48, No. 12, December 2001, pages 1444-52, which is expressly incorporated herein by reference. This method involves determining the location of an ultrasound transducer, which is to be carried by a catheter to be tracked, within the coordinate system of the 3-D image. The main limitation of this method is that, because it reports the location of the transducer within the coordinate system of the 3-D image, the coordinate system will change as the position of the imaging device changes, and thus, any previously registered mapping data and ablation locations will be lost. This becomes even more crucial if the imaging device is an internal device, e.g., a intracardiac or transesophogeal imaging probe, which is often maneuvered within the body of the patient during the imaging process.
There thus remains a need for an improved system and method for localizing an image within the body of a patient.